Differentiation of pityriasis rubra pilaris from plaque psoriasis by dermoscopy

Background Diagnosing and differentiating pityriasis rubra pilaris (PRP) from other disorders can be a challenging task. Although histopathology remains the gold standard, it may not be feasible at times, especially in children. Being noninvasive, dermoscopy can be of great diagnostic importance in such a scenario. Dermoscopy overcomes the refractive properties of stratum corneum by interface medium or cross-polarization enabling easier visualization of lesions. Objective To study the dermoscopic features of PRP and compare them with dermoscopic features of psoriasis. Patients and Methods Retrospective observational analysis of dermoscopic characteristics of 28 patients with PRP (16) and psoriasis (12). Polarized dermoscopy at 10× magnification was performed and photographs were captured by Apple iPhone 7. The selection of dermoscopic variables was based on preexisting literature on the dermoscopic patterns of the 2 entities. Results The most frequent dermoscopic criteria of PRP were the presence of a central hair (16/16; 100%), follicular plugs (11/16; 68.7%), and perifollicular yellow/orange halos (9/16; 56.2%). No vascular structures were seen in our cases of PRP. Psoriasis was characterized by regularly distributed dotted vessels (12/12; 100%) and white scales (8/12; 66.6%). Conclusions Round-to-oval yellowish areas surrounding a central hair with or without follicular plugs represents the most frequent dermoscopic pattern of PRP.

). Histological diagnosis of PRP and PP was based on the identification of the characteristic features of each disease under microscopic examination. Analytically, alternating orthokeratosis and parakeratosis in both directions, hypergranulosis, irregular acanthosis, thick suprapapillary plates, and sparse-to-moderate lymphocytic perivascular infiltrate in the dermis, and dilated hair follicles filled with a dense, horny plug were criteria for PRP. In contrast, psoriasiform epidermal hyperplasia accompanied by tortuous, dilated capillaries in the superficial papillary dermis and a perivascular mononuclear cell infiltrate were the criteria for the histopathological diagnosis of PP.
Any disagreement in the assessment of morphological criteria and diagnosis between the 2 dermoscopists or pathologists were resolved by consensus. All evaluators were unaware of patients' clinical details.

Results
Overall, 28 lesions from 28 patients were included in the study. The histopathological diagnosis was PRP in 16 patients and PP in 12 patients. The male-to-female ratio was 3.5:1 and the mean age was 18.2 years. As shown in Table 1 Figure 3, Table 1).

Discussion
Our results suggest that the dermoscopic pattern of PRP consists of yellow/orange halos surrounding follicles with a

Introduction
Pityriasis rubra pilaris (PRP) is a relatively uncommon skin disease, clinically characterized by follicular keratotic plugs, red to orange plaques, and palmoplantar hyperkeratosis.
However, in cases with atypical clinical presentation, PRP is sometimes confused with psoriasis, both clinically and histopathologically. In such cases, repeated biopsies may be needed to confirm the diagnosis. The main differential diagnosis of adult-onset PRP is psoriasis.
Dermoscopy has been shown to represent a useful clinical tool for the clinical evaluation of several inflammatory skin diseases and the differential diagnosis among entities with similar clinical manifestations. Available data of the dermoscopic morphology of PRP are scant, whereas evidence on the value of dermoscopy for discriminating PRP from psoriasis is even poorer.
The aim of the present study was to investigate the dermoscopic findings in a series of patients with PRP as compared with those in a control group of patients with psoriasis.

Patients and Methods
In this retrospective observational study we evaluated the dermoscopic criteria of PRP and psoriasis. Ethics committee approval was obtained. The database of our center was  and previous studies were conducted in populations of different skin types, the dermoscopic pattern of psoriasis appears

Limitations
The number of patients recruited in the study was small and larger scale studies are required. All patients were Indian and interethnic variability could not be evaluated.

Conclusions
Round-to-oval yellowish areas surrounding a central hair with or without follicular plugs represents the most frequent dermoscopic pattern of PRP. In contrast, psoriasis is typified by numerous and regularly distributed dotted vessels and white scales. This information might aid clinicians to differentiate atypical cases of PRP from psoriasis.
one of the consistent findings of PRP, has been suggested as a potent negative predictive factor for the diagnosis of psoriasis. A previous study investigated the accuracy of dermoscopic criteria for the differentiation of psoriasis from other inflammatory diseases. According to the latter study, the presence of regularly distributed dotted vessels is very highly suggestive of psoriasis over any other diagnosis.
Although the latter study did not include PRP in the control group, our results and previous results on dermoscopy of PRP suggest that the same conclusion is very likely to be valid for the differential diagnosis between psoriasis and PRP. In addition, the latter study suggested yellow color as a potent negative predictive factor for the diagnosis of psoriasis. In the present and previous studies, yellow color was one of the most consistent findings of PRP, supporting further that dermoscopy is useful for the differential diagnosis between PRP and psoriasis.
Previous reports investigated also the dermoscopic findings in peculiar subtypes of PRP. Specifically, erythrodermic PRP was suggested to dermoscopically display orange blotches and islands of nonerythematous (spared) skin displaying reticular vessels [4,5]. In contrast, erythrodermic psoriasis is typified by diffusely distributed whitish scales and regularly arranged dotted/glomerular vessels [6].
Keratoderma resulting from PRP has been suggested to display structureless orange areas of different sizes in a patchy distribution along with white scales [4]. Dermoscopy of juvenile PRP also shows multiple whitish keratotic follicular plugs and a yellowish peripheral keratotic ring surrounded by erythema with some linear vessels [7]. Our sample did not include patients with these peculiar forms of PRP, so the latter evidence cannot be assessed by the present study.